1. Field of the invention
The present invention is related to methods of compression of binary digital images. More precisely, it relates to the compression of a pixel matrix depending on the image composition.
2. Prior Art
The Combined Quadtree compression applied to a black and white digital image allows treating each 16×16 zone according to its frequency in order to compress information. By identifying the high and the low frequencies, this method makes it possible to concentrate the majority of the treatment on the zones charged in graphic content. The method includes a tiling of the image in 16×16 pixel matrices (see FIG. 1A). Each 16×16 matrix is then subdivided in four matrices of 8×8 pixels (see FIG. 1B). Each matrix of 8×8 pixels is then subdivided in four matrices of 4×4 pixels (see FIG. 1C). Lastly, each matrix of 4×4 pixels is subdivided in matrices of 2×2 pixels (see FIG. 1D). The second stage consists in evaluating the total value of all the pixels constituting each 16×16 matrix. If all the pixels are black, the bits “00” are recorded. If all the pixels are white, the bits “01” are recorded. Lastly, if some pixels are white and some are black, the bits “10” are recorded. Only the symbols 00, 01 and 10 are used with the Combined Quadtree compression. If the matrix has been identified as being of the type “10”, each 8×8 zone is evaluated according to the same notation. The calculation of the matrix is finished when there is no more “10” symbol to break up. The last stage uses only one bit per symbol, that is to say “0” for a black element and “1” for a white element (see FIG. 2 where “B” means Black, “W” means white and “G” means to be decomposed).
The Quadtree compression makes it possible to reduce the information necessary to represent an image and offers a fast and effective solution to reference a graphic element in a more complex graphical environment.
The principal drawback of the Quadtree method in the compression of black and white digital images is that it treats each tile independently one from another without taking into account the general composition of the treated document and the treatment of the preceding tiles. Moreover, the notation of the symbols at the time of the identification of the zones of the matrix is static and does not take into account the real population of pixels to be treated. Consequently, it is possible for the compression to create a compressed file with a size greater than that of the original document.